Flat panel field emission displays (FEDs), like standard cathode ray tube (CRT) television sets, generate light by impinging high energy electrons on a picture element of a phosphor screen. The excited phosphor then converts the electron energy into visible light. However, unlike conventional television CRTs which use a single electron beam to scan across the phosphor screen in a raster pattern, FEDs use individual stationary electron sources for each pixel of the phosphor screen. Thus, a screen with a million color pixels has at least a million individual electron sources. There are three electron sources, each source consisting of many emitters, for each pixel in RGB color screen; one for red, one for green and one for blue. By using stationary electron sources instead of a scanning beam, the distance between the electron source and the phosphor screen can be made to be extremely small. Consequently, FED displays can be made to be very thin.
As mentioned, conventional CRT displays use electron beams to scan across the phosphor screen in a raster pattern. Specifically, the electron beams scan along a row in a horizontal direction and adjust the intensity according to the desired brightness of each picture element of that row. The electron beams then step in a column (vertical) direction and scan the next row until all the rows of the display screen are scanned. In marked contrast, in FEDs, a group of stationary electron sources are formed for each picture element (pixel) of the display screen. More specifically, the pixels of an FED flat panel screen are arranged in an array of horizontally aligned rows and vertically aligned columns. A portion 100 of this array is shown in FIG. 1. The boundaries of a respective pixel 125 are indicated by dashed lines and in this configuration include a red point, a green point, and a blue point. Three separate row lines 130a-130c are shown. Each of the row lines 130a, 130b, and 130c is a row electrode for one of the rows of pixels in the array. A pixel row is comprised of all the pixels along one row line 130. Each column of pixels may include three columns lines 150: one for red, a second for green, and a third for blue. The column lines 150 control gate electrodes of the FED screen. When electron-emitting elements contained within the row electrode are suitably excited by adjusting the voltage of the corresponding row lines 130 (row cathodes) and column lines 150 (gate electrodes), electrons are emitted and are accelerated toward a phosphor anode 120. The excited phosphors at the anode 120 then emit light.
The row lines 130 are driven by a plurality of row drivers in the display. Each row driver is responsible for driving a group of rows. However, only one row is active at a time across the entire FED flat panel display screen. Therefore, an individual row driver drives at most one row electrode at a time. A supply voltage line is coupled to all row drivers and supplies the row drivers with a driving voltage for application to the row cathodes. During a screen frame refresh cycle (performed at a rate of approximately 60 Hz), one row is energized to illuminate one row of pixels for an "on-time" period. This is typically performed sequentially in time, row by row, until all pixel rows have been illuminated to display the frame. Assuming frames are presented at 60 Hz and the FED display has n rows in the display array, each row is energized at a rate of 16.7/n ms. In a typical display having 480 rows, each row is energized at a rate of 34.8 .mu.s. The brightness of the target phosphor at the anode 120 depends on the amount of time a voltage is applied across the row electrode and the gate (e.g., on-time window). The larger the on-time window, the brighter the pixel will appear to a viewer. Since the rows are energized at a high rate, it is critical to ascertain that each row is energized at exactly the same time after the rows are activated. Otherwise, if some rows have a slightly longer "on-time" than the others, the brightness across the screen will not be uniform which can cause unwanted screen artifacts.
Unfortunately, in prior art FED systems, it is difficult to ascertain a uniform "on-time" for all the row drivers. The principal reason is attributed to manufacturing complications which cause row drivers to have different settling times. That is, row drivers which settle faster than others activate or deactivate the rows quicker, causing slight discrepancies in the "on-time" among the rows. FIG. 1B illustrates this problem. As shown, the row driver 1 settles at a faster rate than row driver 2, but slower than row driver 3, causing differences in the "on-time" windows among the rows. As a result, bands of uneven brightness appear on the display. A means to cause the row drivers to settle to the same voltage at the same time eliminates this brightness variation problem. One prior art method of matching the settling times of the row drivers fabricates the row drivers from adjacent dice on the same wafer. This solution, however, is not practical because there is no guarantee that row drivers made from the same wafer have the same settling time. Further, if one row driver in a display malfunctions, the whole set of row drivers have to be replaced with others from the same wafer.
Accordingly, the present invention provides a mechanism and device for eliminating objectionable horizontal bands of different brightness on the display. The present invention also provides a mechanism and device for normalizing the settling times of all the row drivers in a FED display. These and other advantages of the present invention not specifically mentioned above will become clear within discussions of the present invention presented herein.